In a reciprocating engine, which is one type of an internal combustion engine, a piston undergoes a reciprocating motion within a cylinder bore provided in a cylinder block. During the reciprocating motion, the outer peripheral surface of the piston and the inner peripheral wall surface of the cylinder bore slidingly contact each other, whereby a frictional force is generated between the outer peripheral surface of the piston and the inner peripheral wall surface of the cylinder bore. The frictional force causes energy loss, and therefore, it is preferred to make the inner peripheral wall surface of the cylinder bore as flat as possible to suppress the frictional force between the piston and the cylinder bore. However, if the inner peripheral wall surface of the cylinder bore is made flat, lubricating oil cannot be retained between the outer peripheral surface of the piston and the inner peripheral wall surface of the cylinder bore, and this tends to cause seizure between the piston and the cylinder bore due to a frictional heat between the piston and the cylinder bore.
Thus, there is known a cylinder block in which the inner peripheral wall surface of the cylinder bore is formed with substantially flat plateau surfaces and groove parts for retaining lubricating oil for the purpose of reducing the frictional force between the piston and the cylinder bore while retaining the lubricating oil between the outer peripheral surface of the piston and the inner peripheral wall surface of the cylinder bore (see Patent Document 1, for example).